Microstructure, phase composition and hardness evolution in 316L stainless steel processed by high-pressure torsion

Abstract

A 316L stainless steel was processed by high-pressure torsion (HPT) to evaluate the grain refinement and phase transformation. The initial material was essentially a single phase γ-austenite with a coarse-grained microstructure of ~42 µm but the grain size was reduced to ~45 nm after 10 turns of HPT. In addition, there was a phase transformation and the initial γ-austenite transformed initially to ε-martensite and finally to α'-martensite with increasing strain. The dislocation density increased to an exceptionally high value, of the order of ~10 16 m -2 , in the main α'-martensite phase after 10 HPT revolutions. The formation of the multiphase nanocrystalline microstructure yielded a four-fold increase in hardness to reach an ultimate value of ~6000 MPa. The Hall-Petch behavior of the HPT-processed alloy is compared directly with coarse-grained materials.

High temperature thermal stability of nanocrystalline 316L stainless steel processed by high-pressure torsion

“…The HPT deformation was carried out for low (N = ¼) and high (N = 10) numbers of revolutions. In an earlier study, the microtructures, the phase compositions and the hardness of these HPT-processed UFG 316L samples were studied in detail [5].…”

“…In addition to grain refinement, a simultaneous phase transformation also occurs during HPT with the transformation sequence γ-austenite → ɛ -martensite → α'-martensite. It was also revealed that the nanocrystalline grains yielded an exceptionally high hardness of ~6000 MPa after 10 turns of HPT [5]. It should be noted that the ultrahigh-strength martensitic steels often suffer from severe intergranular embrittlement which may lead to shorter operational lifetimes.…”

“…In addition, it is used also as a standard construction material in engineering applications and nuclear power facilities due to its superior mechanical properties such as high strength, good ductility, high fracture toughness, excellent corrosion resistance and a low rate of absorption for neutron radiation [3,4]. In practice, 316L stainless steel has been studied extensively in order to further improve the thermal and mechanical properties through tailoring of its microstructure and chemical composition [4][5][6][7][8].…”

“…Highpressure torsion (HPT) is one of the most effective SPD methods for processing ultrafinegrained (UFG) or nanocrystalline microstructures [11] and results have shown that the grain size of 316L can be refined to the nanocrystalline range by HPT processing [5,8,12,13]. In addition to grain refinement, a simultaneous phase transformation also occurs during HPT with the transformation sequence γ-austenite → ɛ -martensite → α'-martensite.…”

“…In an earlier study [5], the evolutions of the dislocation density, grain size and hardness was investigated with increasing numbers of HPT turns at the periphery of 316L steel disks with the number of turns varying between ¼ and 10. It was found that a nanocrystalline microstructure with a very high dislocation density was formed by HPT and simultaneously the initial γ-austenite phase was partly transformed into ɛ -and α'-martensites.…”